[Haskell-cafe] Read Instance for UArray won't port to linux
bringert at cs.chalmers.se
Wed Mar 14 10:27:35 EDT 2007
Simon Peyton-Jones wrote:
> | The error in linux is:
> | Illegal instance declaration for `Read (UArray Int Double)'
> | (The instance type must be of form (T a b c)
> | where T is not a synonym, and a,b,c are distinct type variables)
> | In the instance declaration for `Read (UArray Int Double)'
> | Why does it want three parameters for the instance type? I am baffled by
> | this.
> It's saying that in Haskell 98 you can have
> instance Read (UArray a b)
> (for type variables a, b) but not
> instance Read (UArray Int Double)
> (As Spencer says, use -fglasgow-exts to lift the H98 restriction.)
> However, you mis-read the error message to say "the instance type must have three parameters", which isn't what I meant at all! I was trying to use an example of the general form, but conveyed the wrong idea.
> I can see why you read it that way. Now you understand, can you give me a better error message?
I'm not the original poster, but what about just changing it to "The
instance type must be of the form (T a1 ... an) where T is not a
synonym, and a1 ... an are distinct type variables)"?
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